How is hypothermia classified and what are the corresponding treatment approaches based on core body temperature?

Hypothermia Classification

Hypothermia is classified into distinct severity categories based on core body temperature: mild (32-35°C), moderate (28-32°C), severe (<28°C), and profound (<24°C), with normothermia defined as 37 ± 0.5°C. 1

Standard Classification System

The universally adopted classification system defines hypothermia severity as follows:

• Cold stress: 35-37°C 2, 1
• Mild hypothermia: 32-35°C (or 34-36°C in trauma populations) 3, 1
• Moderate hypothermia: 28-32°C (or 32-34°C in trauma populations) 3, 1
• Severe hypothermia: <28°C (or <32°C in trauma populations) 3, 1
• Profound hypothermia: <24°C 1

Context-Specific Classifications

In trauma patients, a separate classification system has been adopted with slightly different temperature thresholds due to the accelerated heat loss (400 kcal/h versus 60-75 kcal/h in non-trauma patients) and the fact that even mild hypothermia causes significant morbidity in this population. 3 This trauma-specific system defines mild hypothermia as 34-36°C, moderate as 32-34°C, and severe as <32°C. 3

Clinical Significance by Temperature Range

Mild Hypothermia (32-35°C)

• Impaired diastolic relaxation begins at approximately 34.8°C, marking the onset of cardiovascular compromise 2
• Platelet function becomes impaired between 33-37°C, and clotting factor activity begins declining below 33°C 2
• Patients typically present with confusion, uncoordination, tachypnea, tachycardia, hypertension, and increased cardiac output 2
• Shivering thermogenesis is present 2

Moderate Hypothermia (28-32°C)

• Cardiac function transitions from compensatory to depressive 2
• Bradycardia, prolonged PR interval, and Osborne (J) waves appear on ECG 2
• Somnolence develops, with progressive central nervous system depression 2
• Cold-induced diuresis initially occurs, followed by decreased GFR and urine output 2
• Medullary depression causes decreased minute ventilation 2

Severe Hypothermia (<28°C)

• Associated with 84.9% of casualties in multicenter surveys 3
• Profound cardiovascular instability with risk of ventricular fibrillation 1
• Comatose status typically present 4
• Brain death cannot be diagnosed until rewarming to at least 34°C, as severe hypothermia mimics brain death 2

Treatment Approach Based on Classification

Level 1 (Mild Hypothermia: 32-35°C)

Passive and active external rewarming strategies are appropriate for mild hypothermia. 3, 1

• Remove wet clothing immediately and move to warm environment 1, 5
• Cover with two warm blankets 3
• Allow passive rewarming with increased environmental temperature 1
• Provide high-calorie foods or drinks if alert 1
• Monitor temperature every 15 minutes 3, 2

Level 2 (Moderate Hypothermia: 28-32°C)

Active external rewarming combined with warmed IV fluids is first-line treatment for moderate hypothermia. 1, 5

• Continue all Level 1 measures 1
• Apply forced-air warming blankets (e.g., Bair Hugger), achieving rewarming rates of approximately 2.4°C/hour compared to 1.4°C/hour with passive blankets alone 1, 5
• Use heating pads, radiant heaters, or water-circulating warming blankets 3, 1
• Administer warmed isotonic crystalloid (normal saline or Ringer's lactate) at 40-45°C, volumes of 500 mL to 30 mL/kg 5
• Provide humidified, warmed oxygen 1, 5
• Monitor temperature every 5 minutes 3, 2

Level 3 (Severe Hypothermia: <28°C)

Active core rewarming methods are required for severe hypothermia, including invasive strategies such as cavity lavage or extracorporeal circuits. 3, 5

• Continue all Level 1 and Level 2 measures 1
• Activate emergency response system 1
• Consider peritoneal lavage as third-line active core rewarming method 5
• Handle patient gently to avoid precipitating ventricular fibrillation 2, 1
• Continuous cardiac monitoring for arrhythmias is mandatory 2, 1

Critical Temperature Monitoring

Accurate core temperature measurement with a low-reading thermometer capable of measuring below 35°C is crucial for proper classification and treatment. 2, 4

• Preferred methods: Esophageal, bladder, or rectal thermometry 2, 1
• Acceptable alternatives: Tympanic infrared probes when oral measurement is not feasible 1
• Avoid: Axillary measurements, which consistently read 1.5-1.9°C below actual core temperature 1
• Peripheral measurements can underestimate severity by up to 1°C 2

Rewarming Targets and Endpoints

Target a minimum core temperature of 36°C before considering the patient stable, but cease rewarming at 37°C as higher temperatures are associated with poor outcomes and increased mortality. 3, 1, 5

Critical Pitfalls to Avoid

• Never diagnose based on clinical presentation alone: A case report documented a patient with severe hypothermia (25.1°C) who was alert and communicative with vital signs suggesting mild hypothermia, demonstrating that clinical presentation can be misleading. 4
• Do not ignore coagulopathy risk: Even mild hypothermia impairs hemostasis, which is critical in trauma or surgical patients. 2
• Avoid cold IV fluid boluses: These are only indicated for therapeutic hypothermia, not accidental hypothermia. 1
• Monitor for rewarming complications: Including rewarming shock, arrhythmias, hypotension, electrolyte abnormalities, hyperglycemia, and coagulopathy. 3, 1, 5

Mortality Impact

Hypothermia is an independent risk factor for mortality, with mortality rates increasing from 7% in normothermic trauma patients to 43% in hypothermic patients. 3 Reducing the period of hypothermia increases the probability of successful resuscitation. 3